Method and apparatus for introducing controlled spin in optical fibers

ABSTRACT

Optical fiber is provided with a periodically reversing spin while the fiber is pulled through a melt zone. A cooled region of the fiber downstream from the melt zone passes between a pair of opposed elements. The opposed elements are moved so that surface regions engaging the fiber move in opposite lateral directions relative to one another, thus spinning the fiber about its axis. The lateral movement of the engaged surface portions is periodically reversed to reverse the spin direction. The opposed elements may include belts or rollers, which can be tilted to orientations oblique to the longitudinal direction of the fiber.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the manufacture of opticalfibers.

[0002] Optical fiber used in communication systems typically includes acore of glass surrounded by a cladding also formed from glass havingdifferent optical properties from the core. The fiber typically iscovered with a protective outer coating. Such fibers can be made bydrawing a thin strand from a heated, partially molten preform formedfrom glass having the correct composition to make the core surrounded bya layer of glass having the appropriate composition to make thecladding. As a strand of soft, molten glass is pulled from the preform,both the core glass and the cladding glass stretch. The core remains inthe middle and the cladding remains on the outside, thus forming thecomposite core and cladding structure of the finished fiber. As thefiber is pulled away from the preform, it cools and solidifies, and thecoating is applied. These processes are performed at high speeds so thatthe fiber is drawn at high rates.

[0003] In operation of an optical communication system, light applied atone end of the fiber is pulsed or progressively varied in accordancewith the information to be transmitted. The pulses or progressivelyvarying light are received at the other end of the fiber. The speed atwhich light passes along a fiber depends upon many factors including theoptical properties of the materials making up the core and cladding and,the diameter of the core. The fibers commonly used for optical datatransmission systems are so-called “single mode” fibers. In thesefibers, the core diameter is small enough that all of the light mustpass through the core in a so-called “fundamental” or “HE11” mode oftransmission. Full discussion of transmission modes in optical fibers isbeyond the scope of this disclosure. However, the fundamental or HE11mode can be regarded as propagation of light straight along the axis ofthe core, as opposed to higher-ordered modes which can be thought of aspropagation of light in a zig-zag pattern.

[0004] In a theoretically perfect single mode fiber, because all of thelight passes through the fiber in the same mode, all light of a givenwavelength will pass along the length of the fiber with the samevelocity. However, the light passing along the fiber typically includesportions having different polarizations, i.e., different orientation ofthe electromagnetic waves constituting the light. If the fiber core isnot perfectly cylindrical, but instead is out of round so that it haslong and short diameters, light of one polarization will have itselectrical waves aligned with a long diameter of the core whereas lightof the other polarization will have its electrical waves aligned withthe short diameter of the core. In this case, the effective diameter ofthe fiber core will be different for light of one polarization than forlight of another polarization. Portions of light having differentpolarizations will travel at different velocities. Stated another way,the fiber has a “slow” axis in one direction perpendicular to itslength, and a “fast” axis in the other direction perpendicular to itslength.

[0005] Light having a direction of polarization aligned with the fastaxis travels more rapidly than light having a direction of polarizationaligned with the slow axis. As a result, the two polarization modespropagate with different propagation constants (β₁ and β₂). Thedifference between the propagation constants is termed birefringence(Δβ), the magnitude of the birefringence being given by the differencein the propagation constants of the two orthogonal modes:

Δβ=β₁−β₂.

[0006] Birefringence causes the polarization state of light propagatingin the fiber to evolve periodically along the length of the fiber. Thedistance required for the polarization to return to its original stateis the fiber beat length (L_(b)), which is inversely proportional to thefiber birefringence. In particular, the beat length Lb is given by:

L _(b)=2π/Δβ

[0007] Accordingly, fibers with more birefringence have shorter beatlengths and vice versa. Typical beat lengths observed in practice rangefrom as short as 2-3 millimeters (a high birefringence fiber) to as longas 10-50 meters (a low birefringence fiber).

[0008] In addition to causing periodic changes in the polarization stateof light traveling in a fiber, the presence of birefringence means thatthe two polarization modes travel at different group velocities, thedifference increasing as the birefringence increases. The differentialtime delay between the two polarization modes is called polarizationmode dispersion, or PMD. PMD causes signal distortion which is veryharmful for high bit rate systems and analog communication systems.

[0009] This phenomenon is referred to in the art of fiber opticcommunication as polarization mode dispersion or “PMD”. Imperfections inthe fiber other than differences in core diameter can also contribute toPMD. PMD causes distortion of the light pulses or waves transmittedalong the fiber, thus reducing the signal quality and limiting the rateat which information can be passed along the fiber.

[0010] One method of reducing the effects of PMD is to continuallyre-orient the fast and slow axis of the fiber. This can be accomplishedby spinning the fiber as it is drawn, so that the slow axis and the fastaxis of the fiber are repeatedly interchanged along the length of thefiber. Thus, at one point along the length of the fiber the slow axispoints in a first direction perpendicular to the length of the fiber andthe fast axis points in a second direction perpendicular to the lengthof the fiber and perpendicular to the first direction. At another pointalong the length of the fiber, the fast axis points in the firstdirection and the slow axis points in the second direction. In a fiberwith spin, the fast axis traces a generally helical path. The magnitudeof the spin can be expressed as the number of turns per unit length ofsuch helix, i.e., the number of times per unit length of fiber that thedirections of the fast and slow axes interchange. The direction of thespin corresponds to the direction of the helix traced by the fast axis,either right-handed or left-handed. In a fiber with the appropriatespin, the effects caused by the fast and slow axes are substantiallyeliminated and all light travels with the same velocity. To provideoptimum PMD suppression, it is normally desirable to vary the magnitudeand direction of the spin along the length of the fiber.

[0011] Various attempts have been made to impart spin to the opticalfiber during the production process discussed above. For example, asdisclosed in Rashleigh, Navy Technical Disclosure Bulletin, Volume 5,Number 12, December 1980, Navy Tech. Cat. No. 4906, a twisted fiber canbe prepared by rotating the preform about its axis while drawing thefiber from the preform. A similar approach, more generally stated as“continuous relative rotation between the preform and the drawn fiber”is disclosed in International Patent Publication WO 83/00232. Asdisclosed, for example, in U.S. Pat. No. 4,509,968, the processinvolving rotation of the preform leads to considerable practicaldisadvantages. The preform is a massive, soft object which must bemaintained at a high temperature. The '968 patent, therefore, proposesto produce a helical or “chrialic” structure in the fiber by feeding thefiber through a set of nips at the cold or downstream end of the fiberdrawing process while continually spinning the frame holding the nips. Acomplex arrangement of a frame and fiber takeup drum is used in thisprocess to transfer the fiber from the spinning nips to the takeup drum.

[0012] Hart, Jr., et al. U.S. Pat. Nos. 5,418,881 and 5,298,047 discloseanother process for making fibers with spin of alternating clockwise andcounterclockwise directions. In this process, the cold end of the fiberpasses around a roller while the roller rotates about an axisperpendicular to the longitudinal or upstream-to-downstream direction ofthe fiber. The roller is periodically moved so that the fiber tends toroll along the surface of the roller, parallel to the axis of rotationof the roller. The fiber periodically slips or jumps along the surfaceof the roller. Despite these and other efforts in the art, there arestill needs for further improvements in processes for imparting acontrolled spin to an optical fiber. In particular, there are needs forprocesses which can provide non-uniform spin, and particularlyalternating spins in opposite directions to the fibers in a repeatable,controllable manner. There are corresponding needs for reliable,repeatable apparatus for imparting controlled spins to fibers. Inparticular, there are needs for methods and apparatus which can impartappreciable spin to a fiber in a repeatable manner during high speedfiber drawing, and which can be used in combination with conventionalfiber drawing equipment and processes.

SUMMARY OF THE INVENTION

[0013] In view of the foregoing, it is an object of the presentinvention to provide improved methods and apparatus for reducing PMD.More particularly, it is an object of the invention to provide methodsand apparatus for imparting spin to an optical fiber in order to reducePMD.

[0014] One aspect of the present invention includes methods of providingspin in an optical fiber. A method according to this aspect of theinvention desirably includes the step of drawing the fiber so that thefiber moves, relative to a frame of reference, downstream in alongitudinal direction from a melt zone in which the fiber is soft. Thefiber solidifies during this downstream movement. The method furtherincludes the steps of engaging the fiber with surface regions of opposedelements disposed on opposite sides of the solidified fiber downstreamfrom the melt zone and moving these opposed elements so that surfaceregions of the opposed elements engaged with the fiber move withcomponents of velocity, relative to the frame of reference in thedownstream longitudinal direction. The motion is controlled so thatduring at least part of the drawing step, at least one of the surfaceregions moves relative to the frame of reference in a lateral directiontransverse to the longitudinal direction of the fiber movement and sothat the surface regions engaged with the fiber on opposite sides moverelative to one another with opposite components of velocity in lateraldirections to thereby spin the fiber. Most preferably, the moving stepis conducted so that the lateral components of velocity of the surfaceregions relative to one another are repeatedly reversed, so that thefiber is spun repeatedly in alternating, opposite directions.

[0015] According to one aspect of the invention, the step of moving theopposed elements is performed so that the surface region on a first oneof the opposed elements moves in a first surface motion directionoblique to the longitudinal direction of the fiber during at least partof the drawing step. Most preferably, the fiber is forcibly engaged withthe surfaces of the opposed elements.

[0016] According to one aspect of the invention, the step of moving thefirst element includes the step of moving this first element around afirst element axis generally transverse to the longitudinal direction sothat the surface region of the first element engaged with the fibermoves perpendicular to this first element axis. For example, the firstelement may be a roller having a circumferential surface concentric withthe first element axis, and the step of moving the first element mayinclude the step of rotating the roller about the first element axis.The first element may also be a belt and the step of moving the firstelement may include the step of moving the belt around a pulley whilethe pulley rotates about the first element axis. In either case, thestep of moving the first element axis may include the step of rockingthe first element, and the first element axis, about a rocking axistransverse to the longitudinal direction of the fiber and alsotransverse to the aforesaid lateral directions. The rocking axistypically is perpendicular to the first element axis. The surface regionof the first element may be a region on the circumferential surface ofthe roller or on the surface of the belt. When the first element axisrocks about the rocking axis, the direction of motion of this surfaceportion engaging the fiber (the “first surface motion direction”) willsweep through a range of angles with respect to the longitudinaldirection of the fiber. Desirably, this range extends between first andsecond equal but opposite extreme angles. The opposed element may be asimilar belt or roller and bearing on the opposite side of the fiber, atthe same point along the longitudinal direction so that the fiber issqueezed in a nip between the two opposed elements. The second elementmay be moved around a second element axis, and the second element may berocked in substantially the same way about a second element rocking axisparallel to or coincident with the rocking axis associated with thefirst element. Thus, the second surface motion direction swept by theportion of the second element engaging the fiber also sweeps through arange of angles relative to the longitudinal direction. Desirably, theangle between the second surface motion direction and the longitudinaldirection is equal but opposite to the angle between the first surfacemotion direction and the longitudinal direction at all times.

[0017] According to a further embodiment of the invention, the secondelement may include a pair of components, such as a pair of rollers,separated from one another in the longitudinal direction and defining agap therebetween. The first and second elements are engaged with oneanother so that the first element is engaged in longitudinal alignmentwith the gap and so that the first element protrudes into the gap. Thefiber is maintained under tension, as by a takeoff stand disposeddownstream from the first and second elements and the tension of thefiber forces the fiber against the first and second elements. The methodaccording to this aspect of the invention desirably includes the step ofconstraining the fiber against motion in the lateral directions at thecomponents or spaced rollers of the second element. Desirably, therollers include slotted or grooved circumferential surfaces and thefiber is engaged in such surfaces. The second element having the spacedapart components preferably does not move laterally in the fixed orequipment frame of reference. Thus, those portions of the fiberextending upstream and downstream from the first and second elements arenot moved laterally during the process. In this arrangement, the surfaceregion of the first element in contact with the fiber moves back andforth in lateral directions relative to the fixed frame of reference. Ineffect, the fiber rolls around its axis within the grooved surfaces ofthe second element.

[0018] According to yet another embodiment of the invention, the opposedelements may include a pair of rollers having axes transverse to thelongitudinal direction of the fiber, or belts extending around pulleyshaving axes transverse to the longitudinal direction. The step of movingthe opposed elements may include the step of moving each such elementaround its axis while simultaneously translating the elements relativeto the fixed frame of reference, preferably in opposite directions.

[0019] As the fiber spins around its axis between the opposed elements,the spin is transmitted upstream along the fiber and the fiber is spunwithin the melt region, thereby imparting a permanent spin to eachportion of the fiber. Each portion of the fiber acquires a spincorresponding to the direction of spinning motion during the time suchportion of the fiber passed through the melt region and cooled. Thefiber may be collected using conventional take up apparatus such as atakeup reel disposed downstream from the opposed elements. Because thespinning motion of the fiber is repeatedly reversed, the fiber is notplaced under substantial torsional stress on the takeup reel. Becausethe fiber moves with controlled rolling motion on the elements of theapparatus, the process is repeatable and predictable. Essentially anyamount of spin required for desirable optical properties and essentiallyany desired pattern of variation in the degree and direction of spinalong the length of the fiber can be provided.

[0020] Further aspects of the invention provide fiber drawing apparatus.Apparatus according to this aspect of the invention desirably includes astructure defining a melt zone and a solid zone remote from the meltzone as well as means for drawing the fiber along a predetermined pathdownstream in a longitudinal direction relative to the structure so thatthe fiber is substantially molten in the melt zone and solidifies duringdrawing before reaching the solid zone. The apparatus further includes apair of opposed elements as aforesaid disposed in the solid zone andmeans for forcibly engaging the opposed elements with the fiber andmoving the opposed elements during operation of the fiber drawing meansso that surface regions of the opposed elements move relative to oneanother as discussed above.

[0021] Further objects, features and advantages of the present inventionwill be more readily apparent from the detailed description of thepreferred embodiment set forth below, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagrammatic, partially perspective view depictingapparatus in accordance with one embodiment of the invention.

[0023]FIG. 2 is a fragmentary, diagrammatic elevational view along line2-2 in FIG. 1.

[0024]FIG. 3 is a fragmentary sectional view taken along line 3-3 inFIG. 2.

[0025]FIG. 4 is a diagrammatic, partially perspective view depictingapparatus in accordance with a further embodiment of the invention.

[0026]FIG. 5 is a fragmentary sectional view taken along line 5-5 inFIG. 4.

[0027]FIG. 6 is a diagrammatic elevational view depicting portions ofapparatus in accordance with yet another embodiment of the invention,with portions of the apparatus removed for clarity of illustration.

[0028]FIG. 7 is a fragmentary, diagrammatic perspective view depictingapparatus in accordance with a further embodiment of the invention.

[0029]FIG. 8 is a fragmentary, diagrammatic elevational view depictingportions of apparatus in accordance with yet another embodiment.

[0030]FIG. 9 is a fragmentary, diagrammatic elevational view depictingportions of apparatus in accordance with a further embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Apparatus in accordance with one embodiment of the inventionincorporates a furnace 20 adapted to hold a preform 22 of the typecommonly utilized in optical fiber drawing procedures. Furnace 20 ismounted to a frame 24 which defines the fixed frame of reference of thedrawing system. Frame 24, for example, may be the frame of a building organtry in which the fiber drawing operation is conducted. Although smallportions of the frame are illustrated, it should be appreciated that allportions of frame 24 are fixed relative to all other portions of theframe. A takeoff or pulling stand 26 having a pair of opposed drawrollers 28 is provided of furnace 20. Stand 26 includes conventionalelements (not shown) such as electromechanical drive systems for turningrollers 28 about their axes so as to draw a fiber engaged therebetween.A takeup reel 30 is also provided. The takeup reel is also driven byconventional equipment (not shown) in rotation about an axis fixedrelative to frame 24 so as to wind fiber from stand 26 onto the reel.Furnace 20 is arranged to maintain at least a portion of preform 22 in asoft, substantially molten condition. Stand 26 is arranged to pull afiber 32 from the molten portion of preform 22 so that the fiber passesalong a substantially predetermined path. The fiber path has adownstream longitudinal direction L directed along the length of thefiber. References to the longitudinal direction at a given point alongthe upstream to downstream extent of the path should be understood asreferring to the direction along the path at that point. Thus, where thepath is not straight, the longitudinal direction of the path may beoriented differently with respect to the frame 24 at different pointsalong the length of the path.

[0032] In a melt zone 34 adjacent the upstream end of the path, thefiber is substantially molten. However, as the fiber moves downstreamalong the path, it is cooled and solidified so that when the fiberreaches a point 36 considerably downstream from furnace 20, the fiberhas cooled to a substantially solid state. The region of the pathextending from point 36 to the downstream end of the path is referred toherein as the “solid region” of the path. Cooling devices 38 may beprovided between the melt zone and the solid zone. Typically, thecooling device includes a substantial length of frame 24 such that asthe fiber traverses this length of the frame and the correspondinglength along the path, the fiber cools by exposure to the atmosphere.Desirably, the cooling device provides non-contact cooling, such that nosolid object touches the surface of the fiber while it cools.

[0033] A coating device 40 is also mounted to frame 24 in solid zone 36.The coating device is adapted to apply a polymeric coating on theoutside of the fiber. Preferably, the coating device is also anon-contact device. That is, the fiber passes through coat 40 withoutcontacting or engaging any other solid object. Suitable non-contactcoaters are disclosed, for example, in U.S. Pat. No. 4,792,347. Theforegoing elements of the apparatus may be of conventional design ascommonly utilized in the optical fiber drawing art. The apparatus mayfurther include additional guide rollers (not shown) adjacent todownstream end of path 32, for diverting the fiber and hence the pathfrom a straight line and for further constraining the fiber in the path.Other conventional elements such as quality testing equipment and thelike may also be included.

[0034] The apparatus further includes a spin-imparting assembly 42disposed in the solid zone of the path. The spin-forming apparatusincludes an adjustable carriage 46 slidably mounted to frame 24 formovement in cross-path directions X transverse to the longitudinaldirection of path 32. A micrometer adjustment device 48 is provided formoving the carriage in the cross-path directions and for locking thecarriage in place once the same has been adjusted to the desiredlocation relative to frame 24. A yoke 50 is mounted to carriage 46 by ashaft 52 and bearings 54 so that yoke 50 is pivotable relative tocarriage 46 and hence relative to frame 24 about a rocking axis 56extending in the cross-path directions X and intersecting path 32 at apoint of intersection 58.

[0035] Spin-imparting assembly 42 further includes a cylindrical firstroller 60 mounted to yoke 50 for rotation about a first element axis 62.Roller 60 has a circumferential surface 64 encircling first element axis62 and coaxial therewith. The frame of a motor 68 is mounted to carriage46. A crank 66 is supported on the shaft of motor 68 so that the motorcan turn crank 66 around an axis 70 parallel to the rocking axis 56. Aconnecting rod 72 has one end pivotally connected to crank 66 remotefrom its axis 70 and has the opposite end pivotally connected to yoke 50remote from rocking axis 56. Thus, rotation of crank 66 about crank axis70 will drive yoke 50 in a rocking motion about rocking axis 56 betweena first extreme position in which the roller axis or first element axis62 is tilted to the position indicated in broken lines at 62′ in FIG. 1and a second extreme position in which the roller axis or first elementaxis 62 is tilted in the opposite direction to the position shown inbroken lines at 62″ in FIG. 1. As best seen in FIG. 2, extreme positions62′ and 62″ are disposed at equal but opposite extreme tilt angles E1and E2 from a nominal position 62 in which the roller axis or firstelement axis is perpendicular to the longitudinal direction of path 32.In all positions of the roller including these extreme positions,however, the roller axis 62 remains generally transverse to thelongitudinal direction of the path. Desirably, each extreme angle E isbetween about 2 and about 10 degrees from nominal position. As furtherdiscussed below, the desired angle depends upon the desired amount ofspin to be imparted to the fiber. The angles E may be adjusted byadjusting the dimensions of crank 66 and particularly, the spacingbetween the pin joint of connecting rod 72 and axis 70. The speed ofrotation of motor 68 determines the rate at which the yoke 50 and firstelement 60 will rock between the two extreme positions. Motor 68 may bean adjustable speed device such as a stepper motor driven by a digitalcontrol system of the conventional type, a DC motor driven by anadjustable voltage source, an air motor driven by an adjustable gassource, or any other conventional variable-speed motor. Alternatively,motor 68 may be fixed-speed device.

[0036] The spin-imparting assembly 42 further includes a second elementincorporating an upstream roller 76 and a downstream roller 78. Roller76 is mounted to frame 24 for rotation about an axis 80 perpendicular tothe longitudinal direction path 32 and upstream from first element 60whereas roller 78 is mounted to frame 24 downstream from first element60 for rotation about an axis 82 parallel to axis 80 and alsoperpendicular to the longitudinal direction of path 32. Upstream roller76 has a circumferential surface in the form of a surface of revolutionabout axis 80. The circumferential surface includes a generally V-shapedgroove 84 aligned with path 32, so that the fiber passing along the pathis received in the groove. Downstream roller 78 has a similar groove 86,also aligned with the fiber and path.

[0037] As best seen in FIG. 1, the upstream and downstream rollers 76and 78 are spaced apart in the longitudinal direction of the path sothat they define a gap 90 between them. First element or roller 60 isreceived in this gap. That is, the circumferential surface 64 of thefirst rotor extends slightly into the gap in the cross-path direction X.Fiber 32 and hence the path of the fiber bend in the cross-pathdirection through a cross-path deflection angle Ax at each of theupstream and downstream rollers and likewise bends around the firstroller 60 in the cross-path direction. The cross-path deflection angleAx desirably is in the range of about 1 to about 30 degrees, andpreferably between about 5 and about 15 degrees. The cross-pathdeflection angle varies with the setting of carriage 46. Thecircumferential surfaces of the rollers desirably are formed from hard,wear resistant materials such as metals or ceramics. The fiber and thepath are tangent to the first roller at the point of intersection 58 ofrocking axis 56 and path 32. When the first roller 60 is in its nominalposition, the first roller axis 62 is parallel to the axes 80 and 82 ofthe upstream and downstream rollers, and all of these axes extend inlateral directions T, perpendicular to the longitudinal direction L andperpendicular to the cross-path direction X.

[0038] In a process according to one embodiment of the invention,takeoff stand 26 is actuated to pull a fiber along path 32. In theconventional manner, the fiber is taken from preform 22 and elongates inmelt region 34. Each portion of the fiber passes downstream along thepath, cools in cooling region 38 and hence solidifies by the time itreaches point 36. Each region of the fiber is coated on its exteriorsurface with a polymeric coating as it passes through coater 40. Takeoffstand 26 maintains the entire fiber in tension. Because the path is bentin the cross-path direction at rollers 76, 60 and 78, the tension in thefiber has a component in the cross-path direction which tends to holdthe fiber against the rollers. The rollers rotate about their own axesunder the influence of the moving fiber. At the same time, the firstroller 60 and the first roller element axis or first element axis 62rock back and forth about rocking axis 56. As shown in FIGS. 2 and 3, atany instant, a portion 94 of the first roller surface 62 at the point oftangency of the fiber and the first roller, adjacent to intersectionpoint 58, engages one side of fiber 32. Surface portions 96 and 98 ofupstream and downstream rollers 76 and 78 of the second element contactthe opposite side of the fiber. Because rollers 76 and 78 rotate aboutaxes 80 and 82 fixed to frame 24, surface portions 96 and 98 of thesecond element or rollers 76 and 78 always move in a fixed directionrelative to frame 24. Thus, surface portions 76 and 78 always movedownstream in the longitudinal direction L of the path.

[0039] The instantaneous direction of motion of surface portion 94 onthe first element varies with the angle of tilt of roller 60. Whenroller 60 is in its nominal position, with first roller axis 62perpendicular to the longitudinal direction L, the direction of motion Dof portion 94 is also parallel to the longitudinal direction L. However,when roller axis 62 is in the first extreme position 62′ theinstantaneous direction of motion of portion 94 is as indicated at D′,oblique to the longitudinal direction. The instantaneous velocity ofsurface portion 94 relative to the frame of reference 24 thus includes acomponent C′ in a lateral direction T perpendicular to the longitudinaldirection L of the path and perpendicular to the cross-path direction.Again, the surface portions of the rollers 76 and 78 constituting thesecond element have no component of motion in the lateral direction.Thus, as best seen in FIG. 3, surface portion 94 has a lateral componentof motion Ct′ relative to surface portion 96 and also relative tosurface portion 98 (FIG. 2). The fiber thus tends to roll about itsaxis. This rolling motion is transmitted up through the fiber to themelt region 34 and induces a spin in the fiber being formed in the meltregion. The spinning motion occurs in a progressive, rolling motion ofthe fiber on the surface 62 of roller 60. The fiber desirably does notslide or jump along the roller surface. The forcible engagement betweenthe roller surface and the fiber surface provided by tension in thefiber aids in maintaining this rolling motion.

[0040] As the roller tilts back through its nominal position, in whichaxis 62 is at the position indicated in solid lines in FIG. 2, thecomponent Ct′ gradually diminishes. As the roller swings towards theopposite extreme position, the component of motion of surface region 94relative to the frame and relative to surface regions 96 and 98 inlateral direction T reverses direction. Thus, when the roller is tiltedto the opposite extreme position 62″ surface region 94 has a componentof motion in the opposite lateral direction Ct″. Thus, the component ofmotion of surface portion 94 with respect to surface portion 96 and 98in the lateral direction T varies progressively from a maximum magnitudein one lateral direction to a maximum magnitude in the opposite lateraldirection, so that the direction of rotation of the fiber about its axisis repeatedly reversed. Thus, the spin imparted to various portions ofthe fiber varies progressively from a maximum spin in one direction to amaximum spin in the opposite direction and back again. The variation canbe expressed in units of rotation of the fiber about its axis per unitlength along the fiber. The variation in spin per unit length impartedby this method is approximately sinusoidal. The period of the sinusoidalvariation in spin along the length of the fiber is directly related tothe ratio between the linear speed of the fiber passing throughspin-imparting assembly 42 and the frequency of rocking the firstelement of roller 60 about rocking axis 56. The magnitude of the maximumspin and hence the magnitude of the sinusoidal variation and spin isrelated directly to the maximum tilt angle of the roller.

[0041] The spinning motion of the fiber is transmitted toward theupstream end of the path despite engagement of the fiber with roller 76.This result is surprising as the fiber would appear to be wedged in theV-shaped groove 84 and hence difficult to turn. Although the presentinvention is not limited by any theory of operation, it is believed thatthe downstream movement of the fiber facilitates rotation of the fiberwith respect to roller 76 despite engagement of the fiber in theV-shaped groove 84 on the roller surface. To facilitate transmission ofthe spinning motion to the upstream end of the path, the portion of thepath upstream from the spinning device 42 to the melt zone 34 should befree of elements which engage the surface of the fiber. That is, thisportion of the path should be free of rollers and the like engaging thefiber surface. Preferably, the path between melt zone 34 andspin-imparting assembly 42 is substantially straight.

[0042] The fiber rotation is also transmitted downstream, pastdownstream roller 78 into the portion of the fiber extending between thedownstream roller and the stand 26 and takeup roller 30. Because thestand and takeup roller do not rotate about the axis of the fiber, thespinning motion of the fiber would tend to impart an elastic twist tothe fiber in this region of the apparatus. However, because the spinningmotion is periodically reversed, the fiber passes into the stand andtakeup reel with essentially zero twist. The length of the path betweenthe downstream roller and the takeup reel should be longer than theperiod of variation in spin along the length of the fiber so thatelastic twist in the downstream portion of the fiber can be completelyrelaxed before that portion of the fiber is wound onto the takeup reel.

[0043] Apparatus in accordance with a further embodiment of theinvention includes a furnace 120, frame 124, cooling region or apparatus138, and coater 140 similar to the corresponding elements of theapparatus discussed above with reference to FIG. 1. This apparatusfurther includes a takeup reel 130 similar to that described above andassociated drive elements (not shown) for driving the takeup reel. Inthis apparatus as well, the fiber is arranged to pass along apredetermined path 132. The spin-imparting assembly 142 includes a pairof mounts 141 and 143 each mounted to the frame 124 for sliding movementin lateral directions T perpendicular to the longitudinal direction L offiber path 132. A first element carriage 145 is slidably mounted tomount 141 for movement relative to mount 141 in the cross-path directionX perpendicular to the longitudinal and lateral directions. A similarcarriage 147 is slidably mounted to mount 143 for movement in thecross-path directions. A first element or roller 160 has a first elementaxis 162 and a cylindrical circumferential surface 164 encircling thefirst element axis. Roller 160 is rotatably mounted on first elementcarriage 145 so that the first element axis 162 extends substantially inthe lateral directions. The first element or roller 160 has a soft,resilient coating such as rubber of about 30 to about 50 Shore ADurometer about 0.1 mm to about 10 mm thick on a cylindrical body of arigid material such as a metal. A second element or second roller 176has a second element axis 180 and a similar soft, resilient cylindricalcircumferential surface 184 encircling axis 180. The second element isrotatably mounted on second element carriage 147 so that axis 180 alsoextends in the lateral directions, substantially parallel to firstelement axis 162. A drive motor 185 is connected to second element orroller 176 and is arranged to rotate the second element about axis 180at a substantially constant rotational velocity.

[0044] Rollers 162 and 176 are arranged at the same point along thelongitudinal extent of path 132, on opposite sides of the path, so thatthe rollers define a nip 187 therebetween. A micrometer adjustment andlocking device 148 are provided for controlling the position of secondcarriage 147 and hence second roller 176 in the cross-path directions X.First element carriage 145 and first roller 160 are biased by a spring189 in the cross-path direction toward the second roller. An adjustablestop 181 limits movement of the first element carriage 145 in thecross-path direction. This stop assures that the distance betweenrollers 160 and 176 will always be at least equal to a predeterminedminimum, and thus assures that the rollers will not crush the fiber. Ifthe diameter of the fiber is slightly greater than this predeterminedminimum distance, the carriage will remain engaged with the stop. Theresilient circumferential surfaces 164 and 184 of the rollers willcompress slightly, and the fiber will be forcibly engaged with bothcircumferential surfaces. If the fiber diameter substantially exceedsthe predetermined minimum, first roller 160 and first element carriage145 will move in the cross-path direction away from second roller 176against the bias of spring 189. In either case, a fiber extending alongpath 132 between the rollers will be engaged forcibly between therollers at nip 187.

[0045] An oscillation drive unit 191 mounted to frame 124 is linked tofirst mount 141. The oscillation drive unit is adapted to move the firstmount in the lateral directions T relative to frame 124. The oscillationdrive unit may include a crank mechanism as discussed above withreference to FIG. 1 together with a drive motor, or else may include anyconventional linear actuator such as a screw or rack and pinionactuator, a hydraulic or pneumatic cylinder with conventionalpressurization and control circuitry or any other conventional actuator.A pivot link 193 is mounted to frame 124 for pivoting movement about anaxis 197 extending parallel to the longitudinal direction of the path.Pivot link 193 is connected by a first rod 195 to the first mount 141.The opposite end of pivot link 193 on the other side of axis 197 isconnected by a second rod 199 to the second mount 143. Link 193 and rods195 and 199 are connected to one another, and to mounts 141 and 143 byconventional pivotable pin joints. The links are arranged so that as thefirst element undercarriage 141 moves one way in a lateral direction T,second element undercarriage 143 moves in the opposite lateral directionat equal speed.

[0046] In a process utilizing this apparatus, the fiber 132 is engagedbetween the rollers 160 and 176 at the nip 187. Motor 185 is actuated toturn roller 176 and pull the fiber in the downstream longitudinaldirection. The bias applied by spring 189 forces the first elementcarriage 145 towards the opposite carriage 147 and hence forces thefirst element or roller 160 into engagement with the fiber and forcesthe fiber into engagement with the opposite, second element or roller176. Linear drive 191 is actuated to move the first mount 141, firstelement carriage 145 and hence the first element or roller 160 in anoscillating, back-and-forth motion, first in one lateral direction andthen in the opposite lateral direction. In the condition illustrated,the first mount 141 and roller 160 are moving with lateral velocity Ct,whereas the second mount 143 and element 176 are moving with an equalbut oppositely directed lateral velocity Ct′. The surface regions 194and 196 of the rollers at the nip which are momentarily engaged with thefiber are likewise moving with equal but opposite lateral components ofvelocity. Both surface regions are also moving downstream due to therotational movement of the rollers about their respective axes. Thus,the surface regions of the rollers in engagement with the fiber aremoving in directions oblique to the longitudinal direction of the path,but with oppositely directed lateral components of velocity. As bestappreciated with respect to FIG. 5, the opposite lateral components ofvelocity of surface regions 194 and 196 cause fiber 134 to spin aboutits axis. As the fiber is pulled through nip 187 by rotation of roller176, the fiber is spun about its axis and here again this spin istransmitted upstream to melt zone 134. Actuator 191 gradually reversesthe direction of mount 141, thus reversing the directions of the lateralcomponents of motion of surface regions 194 and 196 relative to oneanother and reversing the direction of fiber spin. In the apparatus ofFIGS. 4 and 5, the rollers and motor of the spin-imparting assembly 142act as the takeup stand and draw the fiber through the apparatus. Thus,a separate takeup stand is not required.

[0047] Apparatus according to yet another embodiment of the inventionincludes a furnace 220, frame 224, cooling and coating regions 238 and240, stand 226 and takeup reel 230 similar to the components describedabove with reference to FIG. 1. This further includes a spin-impartingassembly 242 incorporating a first element or roller 260 mounted on ayoke 250. The axis 262 of first roller 260 extends generallytransversely to the longitudinal direction L of the fiber path 232. Yoke250 is pivotally mounted on frame 224, as by a shaft 252 partiallydepicted in FIG. 6. Yoke 250 is pivotable with respect to the frameabout rocking axis 256 extending in the cross-path direction, into andout of the plane of the drawing in FIG. 6. A second cylindrical roller276 having a second roller access 280 is rotatably mounted on a yoke281. Yoke 281 is also pivotable with respect to frame 224 about rockingaxis 256. Rollers 260 and 276 are arranged to form a nip at rocking axis256 so that the fiber is engaged in the nip. As discussed above withreference to FIGS. 4 and 5, the rollers may be biased toward one anotherin the cross-path direction to firmly engage their surfaces with thefiber.

[0048] Yokes 281 and 250 are connected by pin jointed connecting rods283 and 285 to a common link 287. Common link 287 in turn is driven inreciprocating motion by drive unit 291. The links are arranged so thatas common link 287 moves, the yokes 281 and 250 swing about rocking axis256 in oppositely directed pivoting movement. In the position depictedin FIG. 6, each roller is at one extreme of its rocking movement. At theother extreme, the rollers are tilted in the opposite directions fromthose illustrated in FIG. 6. The rocking movement of each roller issimilar to the rocking movement of roller 60 discussed above withreference to FIG. 1.

[0049] The direction of movement of the surface region 294 on roller 260in engagement with the fiber is always perpendicular to the first rolleraxis 262. In the position depicted, the direction of movement D ofsurface region 294 on first roller 260 is oblique to the longitudinaldirection of the path and has a component of movement Ct directed to theleft as seen in FIG. 6. The surface region on second roller 276 engagedwith the fiber is moving with velocity D2, perpendicular to axis 280 andwith component Ct′ in the opposite lateral direction. As in thearrangement of FIGS. 4 and 5, the oppositely directed lateral movementcomponents cause the fiber to twist about its axis. As the yokes 250 and281 pivot about rocking axis 256, the directions of lateral movement ofthe surface regions reverse and hence the spin also reverses. Becausethe components of lateral movement of the surface regions relative toframe 224 are equal and opposite, the lateral movement of the surfaceregions does not tend to displace the fiber in the lateral directions.

[0050] In this arrangement as well, the spinning movement imparted bythe rollers induces a permanent spin in the fiber regions passingthrough the melt zone 234. Here again, because the direction of spin isrepeatedly reversed, the spinning movement does not impart a permanentelastic twist to the fiber downstream from spin-imparting assembly 242.The longitudinal frequency of spin imparted by the apparatus of FIG. 6can be calculated directly from parameters of the system as follows:$\Omega = \frac{\sin \quad \theta}{a}$

[0051] Where:

[0052] Ω is the spin rate expressed in units of angular rotation of thefast-axis direction per unit length along the length of the fiber;

[0053] θ is the angle between each roller axis 262 and 280 and theperpendicular to the longitudinal direction; and

[0054] a is the radius of the fiber.

[0055] Apparatus according to a further embodiment of the invention isschematically depicted in FIG. 7. In this arrangement, the first elementincludes a roller 360 mounted on a yoke (not shown) for rotation aboutthe roller axis 362 and for rocking motion about a rocking axis 356extending in the cross-path directions X. The second element is a belt376 carried on a pair of cylindrical pulleys 377, 378 which in turn aremounted to the frame for rotation about axes 380, 382 fixed relative tothe frame. The roller and belt are biased towards one another in thecross-path directions X and define a nip 387 therebetween, so that afiber 332 passing along the path will be engaged between the surfaces ofthe roller and belt at the nip. In this arrangement as well, the surfaceregion 394 of roller or first element 360 engaged with fiber 332 movesin direction D1 perpendicular to axis 362. Thus, as the axis 362 rocksabout rocking axis 356, the lateral component of motion of surfaceregion 394 falls to zero and then reverses direction in the same manneras described above. The direction of movement of the surface regions onbelt 376 is always parallel to the longitudinal direction. In the samemanner as discussed above, the relative motion of the surface regionsengaging the fiber in opposite lateral directions relative to oneanother causes the fiber to spin about its axis. In the arrangement ofFIG. 7, the motion of roller 360 tends to sweep the fiber laterally.This lateral motion of the fiber can be tolerated, in as much as itreverses direction when the roller axis is tilted in the oppositedirection. However, to prevent transmission of this lateral motionupstream and downstream along the fiber, a pair or grooved constrainingrollers 330 may be provided upstream and downstream from the nip. Theserollers are mounted for rotation around axes fixed with respect to theframe.

[0056] In apparatus according to a further embodiment of the invention,the first element 460 and the second element 476 are both belts. Eachbelt is mounted on a pair of pulleys. The belts are disposed on oppositesides of fiber path 432, so that runs of the belts confront one anotherand define a nip between them. The pulleys associated with the two beltsare mounted to the frame (not shown) for rocking motion about a rockingaxis 456 extending into and out of the plane of the drawing in FIG. 8.Thus, in the position illustrated, the confronting runs of the beltsmove with velocities D1 and D2 directed oblique to the longitudinaldirection of the fiber path. Each belt moves in directions perpendicularto the axes of its respective pulleys. For example, belt 460 moves indirections perpendicular to axis 462 of a pulley associated with thatbelt, whereas belt 476 moves in directions perpendicular to itsassociated pulley axis 480. As the belts and pulleys rock, thedirections of the pulley axes, and hence the directions of theconfronting belt runs swing around the rocking axis. Here again, theconfronting surface portions of the belts or elements 460 and 476 movein opposite lateral directions relative to the frame and hence move inopposite lateral directions relative to one another so as to spin thefiber about its axis. Once again, as the elements and axes tilt or rockaround axis 456, the directions of lateral movement of the elementsrelative to one another reverse and hence the directional spin impartedto the fiber also reverses. As will be readily appreciated, numerousvariations and combinations of the features described above can beutilized without departing from the invention as defined by the claims.Merely by way of example, the crank arrangement illustrated in FIG. 1can be replaced by any other suitable device capable of providing acontrolled rocking motion to rocker 50. For example, electrical,hydraulic and even pneumatic actuators can be employed to impart therocking motion. Similarly, the particular linear drive 191 utilized inthe arrangement of FIG. 4 can be replaced by other actuating devices.The linkage utilized in FIG. 4 to assure equal but opposite linearvelocities can be omitted, and each mount 141, 143 can be provided withan independent linear drive. These linear drives can be controlled toprovide equal but opposite velocities. Alternatively, the rollers can betranslated with unequal velocities. During such unequal translationmovement, the fiber will tend to wander slightly in the lateraldirection along the surfaces of the rollers. Likewise, independentdrives can be used to drive the rockers 250, 281 of FIG. 6independently. In the arrangements discussed above, the drives areoperated gradually, so as to gradually reverse the relative lateralmovement of the surface regions in contact with the fiber and therebygradually reverse the spin imparted to the fiber in a substantiallyperiodic, preferably sinusoidal fashion. However, other patterns ofvariation in the lateral velocities of the surface regions can beemployed. For example, the relative lateral velocities can be brought toa first value and maintained at that value as, for example, bymaintaining constant tilt angles by roller 60 or of the two rollersdepicted in FIG. 6, or by maintaining a constant translational velocityof each roller 160, 176 in FIG. 4 for a prolonged period. This willimpart a substantially constant spin to the fiber during such period.The relative lateral velocities are then reversed and the opposite spinmay be imparted to the fiber. Conversely, the spin in the fiber can bevaried rapidly, in an impulse-like manner.

[0057] Essentially any amount of spin per unit length, and any patternof variation in the magnitude and direction of spin over the length ofthe fiber desired for optical performance can be imparted by methods andapparatus according to the invention. Desirably, the apparatus isdesigned for minimum inertial resistance to movement of the componentswhich must move to change the spin rate. For example, to achieve achange in spin rate, the translating rollers and the associatedcarriages of the embodiment depicted in FIG. 4 must be accelerated.These components should have the lowest possible mass. Particularlypreferred patterns of variation in spin per unit length are disclosed inthe copending, commonly assigned U.S. Provisional Patent ApplicationSerial No. 60/010376 filed Jan. 22,1996, entitled Frequency andAmplitude Modulated Fiber Spins for PMD Reduction and naming asinventors D. Henderson, Ming-Jun Li, D. Nolan and G. Washburn (“the '376application”) the disclosure of which is hereby incorporated byreference herein. A copy of the '376 provisional application is appendedto the present provisional application as appendix A. As disclosed inthe aforesaid '376 application, the spin varies between extremes ofopposite direction according to a periodic function such as a sinusoidalfunction, and the periodic function is modulated (varied) along thelength of the fiber so that. For example, the amplitude of the periodicfunction, the frequency of the periodic function or both may vary alongthe length of the fiber.

[0058] Moreover, the spin-imparting apparatus can be controlled withreference to the actual fiber drawing speed and/or actual fiber diameteras disclosed in the copending, commonly assigned U.S. Provisional PatentApplication Serial No. 60/012290 filed Feb. 26, 1996, entitled Methodand Apparatus for Providing Controlled Spin in Optical Fiber and namingas inventor Robert M. Hawk (“the '290 application”) the disclosure ofwhich is hereby incorporated by reference herein. A copy of the '290application is appended hereto as Appendix B. As described therein, theactual draw speed and actual fiber diameter are measured and the angularposition of a fiber-engaging element such as a roller in aspin-imparting apparatus is set to the angle which will provide thedesired instantaneous spin rate based on the actual draw speed andactual fiber diameter.

[0059] According to further variants, the spring 189 (FIG. 4) used tobias the rollers toward one another may be replaced by any othersuitable bias device such as a pneumatic, electrical or weight-operateddevice. Where the rollers or belts defining a nip have some resilience,these rollers or belts can be mounted at fixed distance from oneanother, so that the fiber is forcibly engaged with the surfaces of thenip defining elements. Also, in embodiments using a rocking roller orbelt to form one or both elements of a nip, the rocking roller or beltcan be forcibly driven in the same way as roller 176 (FIG. 4) so thatthe rocking roller and the opposite nip-defining element form the takeupstand and pull the fiber through the apparatus. Also, the invention canbe applied in processes other than pulling from a preform. For example,a preexisting fiber can be momentarily softened in a melt zone and spunin the same manner.

[0060] Apparatus according to a further embodiment (FIG. 9) incorporatesa first roller 560 and second roller 576 similar to the opposed rollersdiscussed above with reference to FIG. 4. However, rollers 576 and 560are offset from one another in the general longitudinal direction L ofthe fiber path 532 (the longitudinal direction in those sections of thepath upstream and downstream from the rollers). Thus, a plane 561extending between the axes of the rollers is oblique to the generallongitudinal direction of the fiber path. The fiber path bends in thecross-path direction X at each roller, so that the fiber wraps partiallyaround each roller 560 and 576. In other respects, the apparatus issimilar to that described above with reference to FIG. 4. For a givenamount of translational movement of the rollers, the apparatusincorporating offset rollers as illustrated in FIG. 9 typically canprovide a greater degree of spin per unit length than apparatus asillustrated in FIG. 4. However, the degree of spin per unit lengthimparted by the apparatus of FIG. 9 is less predictable than the degreeof spin per unit length imparted by the apparatus of FIG. 4.

[0061] Constraining rollers similar to rollers 330 (FIG. 7) can beutilized in spin-imparting assemblies according to the other illustratedembodiments, to limit lateral movement of the fiber. For example, suchconstraining rollers can be positioned upstream and downstream from therollers depicted in FIG. 4, or upstream and downstream from the rollersdepicted in FIG. 6. According to yet another embodiment of theinvention, the apparatus of FIG. 1 may be modified to replace therocking motion of first element or roller 60 with translationalmovement. Thus, the first element or roller axis 62 may remainperpendicular to the longitudinal direction of the path, and roller 60may be move back and forth in the lateral directions in a manner similarto the movement of roller 160 in FIG. 4.

[0062] As these and other variations and combinations of the featuresdescribed above can be utilized without departing from the presentinvention, the foregoing description of the preferred embodiments shouldbe taken by way of illustration rather than by way of limitation of theinvention as defined by the claims.

What is claimed is:
 1. A method of providing spin in an optical fibercomprising the steps of: (a) drawing the fiber so that the fiber movesrelative to a frame of reference in a downstream longitudinal directionfrom a melt zone wherein the fiber is soft and solidifies during suchdownstream movement; (b) engaging the fiber with surface regions of apair of opposed rollers disposed on opposite sides of the solidifiedfiber at a nip downstream from said melt zone, said rollers havingroller axes substantially perpendicular to the longitudinal direction;(c) rotating said rollers about said roller axes; and (d)translationally moving said opposed rollers relative to said frame ofreference with opposite velocities in lateral directions transverse tothe longitudinal direction of the fiber during at least part of thedrawing step to thereby twist the fiber.
 2. A method of providing spinin an optical fiber comprising the steps of: (a) drawing the fiber sothat the fiber moves relative to a frame of reference in a downstreamlongitudinal direction from a melt zone wherein the fiber is soft andsolidifies during such downstream movement; (b) engaging the solidifiedfiber with a first roller disposed on a first side of the solidifiedfiber at a first longitudinal location, said first roller having a firstroller axis transverse to the longitudinal direction; (c) engaging thesolidified fiber with a pair of second rollers disposed on a second sideof the fiber at locations upstream and downstream from said firstlocation so that the first roller is longitudinally aligned with a gapbetween the second rollers, said second rollers having second rolleraxes transverse to the longitudinal direction; (d) maintaining thesolidified fiber under tension so that the fiber bears on the first andsecond rollers; (e) rotating said rollers about said roller axes; and(f) rocking said first roller relative to said frame of reference abouta rocking axis transverse to the longitudinal direction of the fiber andtransverse to the first roller axis during at least part of the drawingstep so that said first roller axis tilts from perpendicular to thelongitudinal direction, whereby said rotation of the rollers will twistthe fiber.
 3. A method as claimed in claim 2 wherein each of said secondrollers has a circumferential surface defining a groove and saidcircumferential surface constrains the fiber against movement in saidlateral directions.
 4. A method as claimed in claim 2 wherein saidrocking step is performed so as to tilt the first roller axisalternately in opposite directions from perpendicular to thelongitudinal direction.
 5. Optical fiber drawing apparatus comprising:(a) a structure defining a melt zone and a solid zone remote from saidmelt zone; (b) means for drawing a fiber along a predetermined path in adownstream longitudinal direction relative to said structure so that thefiber is substantially molten in said melt zone and solidifies duringdrawing before reaching said solid zone; (c) a first roller disposed ona first side of the path at a first roller location in said solid zone,said first roller being rotatable relative to said structure about saidfirst roller axis; (d) a pair of second rollers disposed on a secondside of the path at second roller locations upstream and downstream fromsaid first roller location, said second rollers defining a gap betweenthem, the first roller being longitudinally aligned with said gap, saidsecond rollers being rotatable relative to said frame about secondroller axes having fixed orientation parallel to one another andtransverse to the longitudinal direction of said path; (e) means forsupporting said first roller on said structure so that said first rolleraxis is oblique to said longitudinal direction at least some timesduring operation of said means for drawing said fiber, said means fordrawing said fiber maintaining the fiber under tension so that the fiberwill bear on said first and second rollers whereby rotation of saidfirst and second rollers will twist the fiber.
 6. Apparatus as claimedin claim 5 wherein said means for supporting said first roller includesmeans for rocking said first roller relative to said structure about arocking axis transverse to the longitudinal direction of the path andtransverse to the first roller axis so that said first roller axis tiltsthrough a range of positions.
 7. Apparatus as claimed in claim 5 whereinsaid means for rocking is operative to tilt the first roller axisalternately in opposite directions from perpendicular to thelongitudinal direction.
 8. Optical fiber drawing apparatus comprising:(a) a structure defining a melt zone and a solid zone remote from saidmelt zone; (b) means for drawing a fiber along a predetermined path in adownstream longitudinal direction relative to said structure so that thefiber is substantially molten in said melt zone and solidifies duringdrawing before reaching said solid zone; (c) a pair of opposed rollersdisposed on opposite sides of the path and defining a nip in said solidzone, each of said rollers having a roller axis transverse to thelongitudinal direction and a circumferential surface encircling theroller axis, each said roller being rotatable about its roller axis; (d)means for supporting said rollers on said structure and moving saidrollers relative to one another in opposite lateral directionstransverse to the longitudinal direction at least some times duringoperation of said drawing means; and (e) means for forcibly engagingsaid circumferential surfaces of said rollers with a fiber drawn alongsaid path by said fiber drawing means, whereby rotation of said rollersabout said roller axes will cause the fiber to twist.
 9. Apparatus asclaimed in claim 8 wherein said means for supporting and moving isoperative to move both of said rollers simultaneously relative to thestructure in opposite lateral directions.
 10. Apparatus as claimed inclaim 8 wherein at least one of said rollers has a resilient layerdefining its circumferential surface and wherein said means for forciblyengaging includes said resilient layer.